Question

Which of the following is/are passive remote sensing technique(s)?

• A. Sonar
• B. Radar
• C. Aerial photos
• D. Landsat images

Hint:

Active sensors send out their own signal and read the echo (sonar, radar); passive sensors only record energy already present, like sunlight (aerial photos, Landsat).

Answer 1

The Correct Option is C, D

Step 1: Understanding the Concept:
Remote sensing means collecting information about the earth's surface without touching it, using a sensor placed on an aircraft or satellite. Every remote sensing system is classed as either active or passive, based on where the energy it measures comes from.

Step 2: Key Formula or Approach:
An active sensor carries its own transmitter, it sends out a pulse of energy (sound or electromagnetic waves) toward the target and then records the echo that bounces back. A passive sensor carries no transmitter at all, it only records energy that already exists in the surroundings, mainly sunlight reflected off the ground or heat naturally emitted by the ground.

Step 3: Detailed Explanation:
(A) Sonar sends out sound pulses through water and measures the time taken for the echo to return, so it is an active technique, not passive.
(B) Radar (Radio Detection And Ranging) transmits its own microwave or radio pulses and measures the reflected signal, so it is also active, not passive.
(C) Aerial photos are taken by a camera mounted on an aircraft that simply records sunlight already reflecting off the ground below, it emits nothing of its own, so this is passive.
(D) Landsat images come from multispectral scanners on Landsat satellites that record sunlight reflected from, and heat naturally emitted by, the earth's surface, again with no signal transmitted by the satellite itself, so this is also passive.

Step 4: Final Answer:
The passive techniques among the given options are Aerial photos and Landsat images, so the answer is 3,4.